1. Field of the Invention
The present invention relates to a semiconductor optical amplification device and an optical integrated circuit, and more particularly, to a semiconductor optical amplification device an active layer of which has a quantum-well structure, and an optical integrated circuit using the semiconductor optical amplification device.
2. Description of the Related Art
Along with rapidly increasing needs of communication in recent years and continuing, information transmission devices of high speed and large capacity are more and more required. A photonic network employs optical time division multiplexing, wavelength division multiplexing, and optical code division multiplexing techniques to process light waves, and is able to transmit information of large capacity at high speed through optical fibers. In addition, a photonic gateway technique is being developed which enables direct transmission of optical signals between photonic networks of different scales and different signal formats without the necessity of converting the optical signals into electronic signals. Because of the photonic gateway technique, it is expected that the processing speed will be boosted up, and communications at even higher speeds will be realized.
Semiconductor optical amplifiers are used in the photonic network and the photonic gateway, and play an important role because of compactness and high performance thereof. Additionally, the semiconductor optical amplifiers are important because they can be formed on the same semiconductor substrate with a laser or an optical modulator.
When using a semiconductor optical amplifier in the photonic network, basically, it is required that the semiconductor optical amplifier have a gain in a wide wavelength range, and the gain have little wavelength dependence. In order to meet this requirement, a semiconductor optical amplifier has been proposed in which an active layer has a multiple quantum well structure.
The active layer of the multiple quantum well structure possesses a step-like density function, and includes a flat gain spectrum in a wide wavelength range compared to a bulk active layer in the related art. Further, the active layer of the multiple quantum well structure has a small differential gain at a longer wavelength region relative to its maximum gain wavelength, thus, high saturation light output is obtainable.
Usually, the bandwidth of an ASE (Amplified Spontaneous Emission) spectrum amplified by the semiconductor optical amplifier is used as an index for describing the band of the gain spectrum of the semiconductor optical amplifier. The ASE spectrum PASE(λ) can be expressed as below by using the gain spectrum G(λ).PASE(λ)=(G(λ)−1)nsp(eff)hνdν
where, nsp(eff) is an effective inversion parameter, hν is photon energy at the observed wavelength, and dν is an observed ASE bandwidth.
In an ideal semiconductor optical amplifier at highly current injected condition, the effective inversion parameter nsp(eff) becomes 1 in a wide wavelength range, thus the wavelength dependence is negligible. The shape of the gain spectrum G(λ) has very close relation to the ASE spectrum PASE(λ).
From the above descriptions, it is specified that a bandwidth between two wavelengths corresponding to a value of the light intensity equaling −3 dB relative to a peak value of the light intensity of the ASE spectrum PASE(λ) is used as an index to describe the band of the gain spectrum of the semiconductor optical amplifier. This bandwidth is referred to as “3 dB bandwidth”.
In the semiconductor optical amplifier having an active layer of a multiple quantum well structure, a gain coefficient of the active layer depends on the guide mode, and hence, a light confinement coefficient changes along with the guide mode. For this reason, the gain of the semiconductor optical amplifier, which is influenced by the product of the gain coefficient and the optical confinement coefficient, is different between different polarization modes; specifically, the gain of the semiconductor optical amplifier is different between the TE mode and TM mode. The difference is referred to as “polarization-dependent gain (PDG)” (hereinafter, referred to as PDG). Because the polarization-dependent gain is not desirable in practical use of the semiconductor optical amplifier, some methods have been proposed to reduce this gain difference of semiconductor optical amplifiers.
A semiconductor optical amplifier able to reduce the polarization dependence of the gain has been proposed, in which an active layer is formed by alternately stacking a tensile strained In0.28Ga0.72As barrier layer and un-strained In0.53Ga0.47As well layer on an InP substrate. For example, reference can be made to K. Magari et al., IEEE J. Quantum Electronics, vol. 30, No. 3, p. 695-702(1994) (hereinafter, referred to as “reference 1”).
In the active layer having such a multiple quantum-well structure, because of the tensile strain applied on the barrier layer, a band structure of the barrier layer is modified. Specifically, energy levels of light holes are higher than energy levels of heavy holes in the valence band of the barrier layer. Due to this, stimulated emission by recombination of the light holes and electrons occurs, and primarily the gain of the TM mode is generated in the barrier layer. On the other hand, in the well layer, similar to common multiple quantum-well structures, primarily the gain of the TE mode is generated.
By controlling the tensile strain applied on the barrier layer and the thicknesses of the layers in the semiconductor optical amplifier, it is possible to reduce the PDG.
Further, a well layer formed from an InGaAsP film is disclosed in A. Godefroy et. al., IEEE Photonics Technology Letters, vol. 7, No. 5, p. 473-475(1995) (hereinafter, referred to as “reference 2”), and A. Ougazzaden et al., Electronics Letters, vol. 31, No. 15, p. 1242-1244(1995) (hereinafter, referred to as “reference 3”).
Further, semiconductor optical amplifiers able to reduce the polarization dependence of the gain are disclosed in reference 2 and reference 3. In these semiconductor optical amplifiers, the active layer is formed by alternately stacking an InGaAs barrier layer and a InGaAsP well layer. A tensile strain is applied to the InGaAs barrier layer, and a compression strain is applied to InGaAsP well layer.
Similar to the semiconductor optical amplifier disclosed in reference 1, the semiconductor optical amplifiers disclosed in reference 2 and 3 are also able to reduce the polarization dependence of the gain, that is, the PDG.
However, in the semiconductor optical amplifier disclosed in reference 1, the 3 dB bandwidth of the ASE spectrum is only 55 nm in the TE mode and 70 nm in the TM mode, and they are not sufficient for realizing a wide bandwidth.
Further, in references 2 and 3, the well layer is formed from an InGaAsP film. As described with respect to FIG. 1, such a structure suffers from some problems.
FIG. 1 is a diagram illustrating a band structure of the active layer of a semiconductor optical amplifier in the related art, specifically, as those shown in references 2 and 3.
As shown in FIG. 1, because of phosphorous (P) existing in the well layer, the energy gap of the well layer increases, and the electron quantum level goes higher. As a result, the energy difference between the electron quantum level of the well layer and the lower compression strain of the conduction band of the barrier layer ΔEc (this is also referred to as “effective barrier height”) is not sufficiently large. For example, in reference 3, the effective barrier height ΔEc is 10 meV, which is even less than the thermal energy at 27° C. (26 meV). Hence, it is thought that many electrons leak out from the quantum well due to the thermal energy and stay in the barrier layer. The electrons in the barrier layer combine with the light holes or the heavy holes, and this produces stimulated emission. This stimulated emission generates a gain spectrum of a narrow bandwidth, similar to the bulk active layer in the related art. For this reason, such kind of semiconductor optical amplifier cannot produce a gain spectrum of a sufficiently wide bandwidth.